Known from the state of the art, e.g. from DE 102004054118 B4, are heat meters; which measure the temperature in the feed line and in the drain line of a heat exchanger and calculate the temperature difference between the feed line and the drain line. Moreover, the amount of a heat transport medium, which is most often a liquid, which has flowed through the heat meter, is determined. From the temperature difference as well as the measured amount of liquid, the energy used is calculated. This used energy can then be billed to a consumer using a certain tariff.
Currently, for registering the temperature in the feed line as well as in the drain line, temperature sensors, which have a temperature dependent resistance, are often applied as the measuring transducer. These are known as RTDs.
In such case, it is provided that the temperature sensors are paired, i.e. the resistance temperature characteristic lines are approximately equal. For such purpose, the characteristic lines of various temperature sensors are compared with one another. If, in such case, the characteristic lines of the two temperature sensors approximately match, then the two sensors can be used in a heat meter. Through temperature sensors paired in this manner, a high measurement accuracy can be assured.
A heat meter can be designed as a compact device, i.e. the computing means is fixedly connected to a housing flowed through by the heat transport medium, or as a combination device with a separate computing means, which is connected with the flow sensor and the temperature sensors by electrical cable.
A heat meter of the type described can also be used as a cold meter for application in a cooling system. For a cold meter, this means that, in the heat transport medium, the lower temperatures are in the feed line and the higher temperatures are in the drain line.
As mentioned above, the temperature sensors used in a heat meter are paired according to the requirements of the DIN EN 1434 standard. This, however, has the disadvantage that, in the case of a defect or replacement for any other reason, the temperature sensors cannot be exchanged individually but must be replaced only in pairs.
The requirements for officially verifiable heat meters are described in the standards MID 004 and EN 1434. Therein the use of liquid heat transport media is prescribed. However, heat meters can also be used for energy measurement of vapor or gaseous media in non-officially verifiable applications.
Additionally, only one transfer function, which is used for the two temperature sensors, is stored in the computing means for determining the respective temperatures. In general the transfer function reflects the relationship between an input variable and an output variable. In the case of a temperature sensor of a heat meter a measurement signal of the temperature sensor is transmitted to the computing means. The measurement signal, in such case, serves as the input variable from which the output variable, i.e. the temperature associated with the measurement signal, is determined by means of the transfer function. Since, however, the respective temperature resistance characteristic lines do differ from one another, in spite of the paired temperature sensors, errors in determining the amount of heat do unavoidably occur. It is known from the state of the art to use a set of standard parameters, as given, for example, in EN 1434-3, in order to describe the transfer function.